1. Field of the Invention
This invention relates to three-cone sealed bearing rock bits having at least three nozzles to enhance removal of rock chips from a borehole bottom.
More particularly, this invention relates to three-cone sealed bearing rock bits having three variable length nozzle bodies extending adjacent the cutting end of the bit; the variable length nozzles enhancing a crossflow of fluid to accelerate the removal of rock chips from a borehole bottom.
Rotary cone rock bits are primarily used in drilling for oil. In the drilling operation, as the drillstring rotates and the cutters on the borehole bottom thereof rotate relative to the bit, parts of the rock formation are cut or broken away. Drilling "mud" is circulated down through the drillstring or stem. The mud enters a chamber formed in the bit body and is directed against the bottom hole formation. The mud passes by the cutters to clear the cutter teeth of debris and picks up cuttings from the formation bottom and circulates them upwardly around the drill stem to the surface.
It has been found advantageous to extend circulation nozzles for the drilling mud downwardly, close to the bottom of the hole being drilled in the formation, to provide better scarifying and suspension of the formation particles in the mud for circulation upwardly and removal from the hole.
2. Description of the Prior Art
There are many patents describing nozzle means to accelerate fluid or mud towards the borehole bottom to accelerate removal of rock chips during operation of a rock bit in an earth formation.
For example, U.S. Pat. No. 1,945,258 describes a two-cone rock bit having water courses that are used primarily to prevent the jet velocity of the mud exiting the water courses from impacting the cutting structure on each of the two cones. This patent does not direct itself toward the enhancement of a crossflow of mud to accelerate removal of rock chips or detritus from the borehole bottom.
U.S. Pat. No. 2,045,368 describes, again, a two-cone rock bit with nozzles of equal distance and length. There is also a center jet in the dome of the rock bit. The center jet is normally used to prevent the rock bit from "balling" during bit operation. Balling occurs when detritus packs between the cones in the center of the bit. This patent, like the foregoing patent, does not teach a crossflow of fluid on the floor of the earth formation to enhance movement of detritus from the floor.
U.S. Pat. No. 2,710,741 describes a two or three-cone rock bit having a reverse flow wherein fluid escapes the drillstring above the pin end or threaded end of the rock bit to direct fluid toward the surface on the outside of the drillstring, thus enhancing removal of rock chips from the borehole bottom. There is, however, no means on the rock bit itself to enhance crossflow of fluids on the borehole bottom.
U.S. Pat. No. 4,068,731, assigned to the same assignee as the present invention, teaches a rotary drill bit with downwardly extended nozzle means, an outwardly facing portion of which serves as a bit stabilizer. The upper portion of the nozzle means is on a greater diameter than the lower portion thereof so that the lower portion is positioned radially inwardly from the borehole wall of the formation. The upper portion of the nozzle means is machined with a larger diameter than the lower portion--the upper portion being provided with hardened tungsten carbide flush-type inserts so that the upper portion of the nozzles will ream the borehole during bit operation.
This patent teaches a two-cone rock bit with a pair of nozzles extended toward the borehole bottom. The pair of nozzles are equal in length and direct fluid at the same rate toward the borehole bottom to remove rock chips from the borehole bottom.
This patent, however, does not teach a means to create a disturbance or crossflow of drilling mud to enhance more efficient removal of detritus from the borehole bottom.
U.S. Pat. No. 4,187,921, also assigned to the same assignee as the present invention, describes a means to enhance the crossflow of fluid to remove detritus from the borehole bottom. This invention teaches the use of one or more cavitation-inducing nozzles in combination with a conventional nozzle of a rock bit. The cavitation nozzle enhances the drilling rate by rapidly removing cuttings from the hole bottom. Cavitation from a cavitating nozzle positioned on one side of the bit reduces the pressure, thereby inducing drilling mud at higher pressure, passing through an opposing noncavitating nozzle to move across the rock-tooth interface. The resultant crossflow of mud removes the cuttings from the hole bottom.
While the principle taught in this patent is valid, it is sometimes difficult to achieve cavitation of the cavitating nozzles since the flow velocity through the cavitating nozzle and the depth of the bit has to be within fairly precise ranges to induce cavitation.
The present invention creates a crossflow or a circulation in the borehole bottom by utilizing three nozzle bodies of varying length whereby the distance from the exit end of each nozzle from the borehole bottom is different, thus creating different flow velocities at the borehole bottom. The varying flow velocities creates a positive crossflow of fluids across the borehole bottom to more effectively lift detritus or rock chips from the borehole bottom toward the surfce through the annulus created between the outside of the drillstring and the walls of the borehole itself.